Download Lecture 3.1: Human Vision: Colour.

Computational Vision
Lecture 3.1: Human Vision - Colour
Hamid Dehghani
Office: UG38
Biological Vision
• Retinal Processing
• Visual Pathway
• Colour
Rods and Cones
• Rods
– ~120 m
– Extremely sensitive
photosensor
– Respond to a single photon
– Poor spatial resolution as
they converge to same
neuron within retina
• Cones
– ~6 m
– Active at higher light levels
– Higher resolution as Signal
processed by several
neurons
Receptive Field
• Receptive field is the area on
which light must fall for neuron
to be stimulated.
• Note difference between
centre and periphery of field
• As early as 1938, frogs were
seen to have different types of
ganglion cells
• Using Cats, (electrical
recordings from ganglion cells)
it was seen that receptive field
contains a circular centre
surrounded by a ring
Ganglion cells
• Two types: "on-center"
and "off-center".
– On-center: stimulated
when the center of its
receptive field is exposed
to light, and is inhibited
when the surround is
exposed to light.
– Off-center cells have just
the opposite reaction
ON/OFF Cells
• Note
– area of
stimulation /
inhibition
– Rate of signal
firing (rebound)
Visual Pathway
• Vision generated by
photoreceptors in the retina
• The information leaves the eye by
way of the optic nerve
• There is a partial crossing of axons
at the optic chiasm.
• After the chiasm, the axons are
called the optic tract.
• The optic tract wraps around the
midbrain to get to the lateral
geniculate nucleus (LGN)
• The LGN axons fan out through
the deep white matter of the brain
and ultimately travel to primary
visual cortex, at the back of the
brain.
Visible Light
• Humans perceive electromagnetic radiation
with wavelengths 380-760nm (1 nm = 10-9 m)
• f = c/λ
•
•
•
f = frequency (Hz)
λ = wavelength (m)
c = speed of light (2.998x108 ms-1)
• E = hf
•
•
E = Energy (J)
h = Plank’s constant (6.623x1034 Js)
Colour
• Objects selectively absorb some wavelengths
(colours) and reflect others
• Human retinas contain three different kinds of
cones to provide very elaborate form of vision
– Gives ability to distinguish different forms of same
objects
• Fruits
• Camouflage
Colour mixing
• Many forms of colour vision proposed
– Until recently some hard to disapprove
• 1802: Proposed that the eye has three
different types of receptors, each sensitive to
a single hue (Young, a British physicist)
– By the fact that any colour can be produced by
appropriate mixing of the three primary colours.
• This became known as Trichromatic (threecolour) theory.
Trichromatic Coding
Trichromatic Coding
– Explains
– Does not account for colour
blending:
Colour mixing
• Many forms of colour vision proposed
– Until recently some hard to disapprove
• 1930s: Hering (German Physiologist)
suggested colour may be represented in visual
system as ‘opponent colours’
• Yellow, Blue, Red and Green – Primary colours
– Trichromatic theory cannot explain why yellow is a
primary colour
Opponent Process Coding
• Yellow, Blue, Red and Green – Primary colours
– Trichromatic theory cannot explain why yellow is a
primary colour
– Also some colours blend and others do not!
• Bluish green, yellowish green, orange (red and yellow),
purple (red and blue) OK
• Reddish green?? Bluish Yellow??
– Opposite to each other
Opponent Process Coding
• Neurons respond to pairs of primary colours
– Red-Green & Yellow-Blue
• Some respond in centre-surround fashion
• Response characteristics determined by
appropriate ganglion cells connections
Opponent Process Coding
Reading
• Vicki Bruce, Visual Perception, Chapters 1 - 3
• Neil Carlson, Physiology of Behavior, Chapter
3, “Vision”